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Weakness after viral fever
is well known - Post Influenzal Asthenia.
But 'flu' itself is not certain in your case.
Even so, with a history of recent hear attack, it is better to check - that no new heart problem has happened.
Again, the effect of drugs should be considered.
See your doctor / get examined / have and EKG and some basic blood tests / get re-assured.
Wishing speedy recovery